In automation technology, field transmitters are frequently used, which serve for recording and/or influencing process variables. Examples of such field transmitters are level measuring instruments, mass flowmeters, pressure gages, temperature gages etc., which record the corresponding process variables—level, mass flow, pressure and temperature. Process variables are influenced using “actuators” which, in the form of valves, for example, influence the flow of a liquid in a pipeline section.
The field transmitters are generally connected to a central control unit which controls the whole process flow. The central control unit evaluates and monitors the measured values for the various process variables and drives the appropriate actuators accordingly in order to influence the process.
Data are transmitted between field transmitter and control unit on the basis of the known international standards for field buses, such as 4–20 mA current loop, Hart, Foundation Fieldbus, Profibus etc.
Since the field transmitters are usually started up and adjusted in situ, a control device integrated in the field transmitter (in situ control) is provided which permits manual input of data required for smooth operation of the field transmitter. By way of example, these data may be calibration data, parameter setting data or other individual settings. Normally, the control device also includes a display in addition to a manual input facility, said display facilitating the input of data by means of appropriate menu control (multisegment display, matrix display), for example. In addition, the display usually also permits the instantaneous measured value of the process variables to be displayed in graphical or digital form.
Such control devices have various drawbacks.
Since the keyboard and the display of the control device need to be protected against the process conditions (dust, moisture etc.) and also against mechanical influence, additional protective flaps are required on the housing of the field transmitter, and also appropriate seals.
In some cases, the control device also needs to be sealed from the interior of the field transmitter housing. These seals are very costly, particularly in the case of applications where there is a risk of explosion (Ex field).
In addition, the control device needs to be controlled in an appropriate manner by a microprocessor, which firstly costs computer power and also possibly additional energy.
In some cases, the control device needs to be integrated in already existing field transmitter housings. Since there is usually a lack of space in the housings, this is possible only with increased complexity.
The individual parts of the control device, keyboard and display etc. increase the complexity of production and are additionally cost-intensive and prone to error. If the control device fails or malfunctions, the field transmitter needs to be visited and repaired in situ by a service technician.
When considered over the operating time of the field transmitter, the control device is used only extremely rarely, but is nevertheless present in many field transmitters.
In each case, the control device can be used to control only the particular field transmitter in which it is permanently integrated.